The present invention relates to capacitors, and particularly to capacitors having multiple ceramic layers. Such capacitors are useful for storage of electric charge in electronic circuits, e.g., as conventional or surface-mounted components.
Multilayer capacitors are those having multiple layers of laminated dielectric ceramic material separated by electrode material. The advantages offered by such multilayer devices include higher capacitance than conventional devices having two electrodes separated by a single dielectric layer.
Presently available multilayer capacitors are expensive to fabricate, because they are fabricated by stacking premetallized green ceramic layers using various processes that involve a number of critical laminating and indexing steps. The green ceramic layers are typically fabricated from sheets made by tape casting or roll compaction methods. Many currently used processes require the application of costly noble metal electrodes between ceramic layers, because the ceramic and electrode layers must be assembled into a stack before the ceramic is fired and only noble metals, e.g., platinum or palladium alloys, will withstand the high temperature necessary for sintering the ceramic after such assembly. The requirement for such noble metal internal electrode layers contributes to the high cost of currently available multilayer capacitors. Such fabrication processes present problems with indexing and stacking faults and bond delamination. Additionally, the number of ceramic layers in the multilayer capacitor is limited by the number of ceramic layers that can practically be laminated and by the binder burn-out problems associated with the current roll compaction or tape casting ceramic layer fabrication methods.
One disadvantage of the stacking methods described above is that, to achieve the maximum benefit from the multilayer configuration, thin ceramic layers (typically about 10-1000 xcexcm thick) are desired. For a large capacitor, the stacking of many layers is therefore required to achieve the desired overall capacitance, exacerbating the problems of indexing and stacking faults, bond delamination, and reliability.
Another known multilayer capacitor is built up from stacked and laminated green ceramic tapes produced by conventional tape casting techniques. Planar internal cavities are produced within this structure in a multilayer configuration by screen printing the tapes prior to lamination with pads of binder rich ceramic ink. After lamination, the tape casting binder and the binder rich ink are burned away and the parts are sintered, yielding a multilayer structure of dense ceramic layers separated by porous planar ceramic layers in the pattern of the desired interdigitated internal electrodes. The internal electrodes are then formed by back-filling these internal porous layers with molten metal or other conductive material. This process, however, is still hampered by the limitations imposed by the above-described indexing and stacking faults. Also, the number of ceramic layers of the multilayer structure is still limited by the number of ceramic layers that can practically be laminated and by the binder burn-out problems associated with current tape casting ceramic layer fabrication methods.
Accordingly, it is an object of the present invention to provide a multilayer capacitor which overcomes the disadvantages of the prior art.
It is another object of the invention to provide a multilayer capacitor having high capacitance which can be more economically fabricated than those found in the prior art.
It is yet another object of the invention to provide net-shape forming techniques for readily and economically fabricating a multilayer capacitor.
In accordance with these objects, in one aspect the invention is a method of fabricating a multilayer capacitor. The method involves net-shape molding from a mixture of a dielectric ceramic powder material and an organic binder a unitary dielectric ceramic body including a top, four sides normal to the top, and a base interconnecting the sides. First and second cavities are molded into at least one side to divide the ceramic body into a plurality of ceramic layers disposed generally parallel to the top. The first cavities alternate with said second cavities in the ceramic body. Each of the ceramic layers except an uppermost and a lowermost of the ceramic layers is joined at one edge to one ceramic layer adjacent thereto by a first ceramic bridge and at the same or a different edge to another ceramic layer adjacent thereto by a second ceramic bridge. The binder is removed from the ceramic body, and the body is sintered at a temperature and for a time to densify the body to near theoretical density. The first and second cavities are filled with one or more materials to form first and second electrically conductive electrode layers, respectively, each electrode layer being bonded to the ceramic layers adjacent thereto. The first electrode layers are electrically interconnected to provide a first set of electrode layers, and the second electrode layers are electrically interconnected to provide a second set of electrode layers alternating with and electrically isolated from the first set electrode layers in the body.
In another aspect, the invention is a multilayer capacitor including a net-shape molded, densified, unitary dielectric ceramic body having a top, four sides normal to the top, and a base interconnecting the sides. The capacitor further includes a plurality of ceramic layers including an uppermost ceramic layer, a lowermost ceramic layer and one or more intermediate ceramic layers all disposed generally parallel to the top. Each intermediate ceramic layer is joined at one edge to one ceramic layer adjacent thereto by a first ceramic bridge and at the same or a different edge to another ceramic layer adjacent thereto by a second ceramic bridge. The capacitor has plurality of electrode layers including an electrode layer between and bonded to each adjacent pair of ceramic layers, the electrode layers nearly completely separating the adjacent pair of ceramic layers. The electrode layers are divided into a first set of electrode layers and a second set of electrode layers. The first set electrode layers alternates with the second set electrode layers in the body. The electrode layers of each set is electrically interconnected with one another and is electrically isolated from the electrode layers of the other set in the body.
In a narrower embodiment, the plurality of electrode layers of the capacitor further includes a lower electrode layer on the base, and the first set of electrode layers includes the lower electrode layer.
In another narrower embodiment, the first and second electrode layers of the capacitor extend into the ceramic body from, respectively, a first of said sides and a second of said sides opposite said first side. The first electrode layers alternate with the second electrode layers in the ceramic body, and each of the ceramic layers except an uppermost and a lowermost of the layers is joined at one edge to one ceramic layer adjacent thereto by a first ceramic bridge and at an opposite edge to another ceramic layer adjacent thereto by a second ceramic bridge such that the ceramic body has a serpentine cross-section.
In yet another narrower embodiment, the first and second electrode layers of the capacitor extend into a first side [one of said sides], the first electrode layers alternating with the second electrode layers in the capacitor. The ceramic bridges join the ceramic layers at edges along a second side of the ceramic body opposite the first side such that the ceramic body has a comb-shaped cross-section. Each of the electrode layers includes a lug integral therewith and including an electrically conductive material extending into the bridge adjacent thereto over a minor portion of the length of the bridge. The first lugs align with one another, while the second lugs align with one another but not with the first lugs. The lugs are exposed at the second side of the ceramic body. The first lugs are electrically interconnected with one another and the second lugs are electrically interconnected with one another to provide the first and second set, respectively, of electrode layers.